It has been noted that in the combustion chamber of a cylinder piston assembly of an internal combustion engine, the displacement of the flame front in a non-turbulent mixture takes place with insufficient speed. This combustion at too low a speed results in numerous disadvantages among which one would mention, as far as controlled ignition engines are concerned, a tendency to knock at full load, incomplete combustion at small load, decreased performance at any load level of the engine, etc. In addition, if the combustion mixture is not homogenous the combustion is still more uncertain as far as its completeness is concerned and the above-mentioned disadvantages are aggravated.
For many years, one has attempted to make improvements on the combustion chambers in order to eliminate these disadvantages.
Thus, it has been shown that if the piston is provided with a projecting part it is possible to avoid a rise in temperature of the still unburnt portion of the gaseous mixture and to delay in this manner its self-ignition and to reduce its tendencies to knock, or pinging.
Also to improve the combustion, it has been proposed to associate a projecting piston part, forming a partition of the chamber at the upper dead center point and to effect a direct injection into one of the sections of such a chamber. However, this technique has not been used in the industry because it presents the disadvantage of employing expensive direct injection. In addition, the pressure increase in the initial combustion zone is such that after the upper dead center point, an intense projection of the combusted gases toward the second part of the chamber takes place with reflection of these gases on the cylinder head generating an uneven combustion and an overall decreased turbulence since it is without direction.
In another prior device, a momentary partitioning of the combustion chamber at the upper dead center point is accomplished by means of a projecting part of the piston and a cavity in the cylinder head, said projecting part of the piston being provided with an upper opening through which the gases originating from the initial combustion escape in the direction of the center of the chamber. The projecting part, being thin at the location of the above-mentioned opening, represents a very poorly cooled hot spot which is a source of self-ignition of the mixture making its application practically impossible. Moreover, the above-mentioned opening is made very narrow in order to extinguish the flame and therefore results in the complication of re-ignition.
In certain engines of recent design, for the purpose of increasing the compression ratio, a mechanical turbulence is created by the rise of the piston with combustion in the zone of the turbulence. Also, with the homogeneity of the mixture having been reached before the upper dead center point, there is no interest in employing double intake. In addition, since the turbulence zone and the combustion zone contain the exhaust valve, the temperature at the end of the compression is very high and the tendency to knock remains significant which implies the utilization of an overall very lean mixture. Therefore, an increase in the compression ratio can only be achieved to the detriment of the specific power which is of the order of magnitude of diesels. However, the resulting increase in weight and expense is prohibitive.
In other engines, a flat cylinder head cooperates with a piston head which divides the chamber into two zones without communication openings at the upper dead center point.
The absence of an opening creates a higher temperature and a greater tendency for self-ignition and increased knocking. This tendency is more pronounced since this zone contains the exhaust valve. The volume of the initial zone is indeed designed to be small, in order to reduce the gaseous mass which self-ignites, but taking into account that this mass tends to be greater because of the impossibility of expansion before the upper dead center point, it remains nevertheless too great and, in this zone, continues to represent too large a portion for not creating any knocking.
The compression ratio of this engine should, therefore, be lower than the corresponding compression at the self-ignition temperature of a mixture of substantially stoichiometric richness.
Also known is a combustion chamber which is bounded by a piston equipped with a projecting part and a cylinder head provided with a cavity into which a lean mixture and a rich mixture are separately admitted, the latter on the side of the cavity. At the upper dead center point, the chamber is likewise partitioned and a communication opening is established between the zone with the rich mixture and the zone with the lean mixture. This arrangement involving two separate intakes is relatively costly, particularly as far as the arrangement of the piston head is concerned.
Finally, a recent arrangement is mentioned where, particularly during idling, there is admitted into a combustion chamber a mixture which, at the moment of intake, sweeps the chamber with a turning motion. It will be noted that this rotary turbulence is the result of a tangential feeding of the combustion chamber through a narrow opening having the effect of a nozzle, which is in parallel with the main feeding. This device makes it possible to generate gas dynamics before the combustion, promoting the latter. Nevertheless, one can believe that at the end of the compression phase, these dynamics are strongly dampened and have only a very small effect on the development and speed of the combustion. In any case, they concern only the operation of the engine when idling.